Metal pipe, and method and device for processing the same

ABSTRACT

A method for processing a metal pipe  1  to make part of the metal pipe  1  in an axial direction thereof into a thick portion  10 , the method includes: forming a shoulder as a stress concentration portion in an axial center of the metal pipe  1  to concentrate a stress thereon; setting the metal pipe  1  in an outer die  20 ; and forming the thick portion  10  by inserting an inner die  25  in the metal pipe  1  to provide space S for forming the thick portion  10  between the inner and outer dies  25  and  20 , and applying the axial pressure to the metal pipe  1  to concentrate the stress on the shoulder to deform the shoulder after the forming and the setting, thereby forming the thick portion  10  from the deformed stress concentration portion as a starting point.

TECHNICAL FIELD

The present invention relates to metal pipes, part of which is madethicker than the other part, and a method and a device for processingthe metal pipes.

BACKGROUND ART

For example, metal pipes have been used in part of exhaust systems ofinternal combustion engines mounted in automobiles. The metal piperepeatedly receives heat of exhaust gas, and stress is concentrated on ajunction between the metal pipe and the other component due tovibrations of the internal combustion engine and a vehicle body. Thus,the metal pipe, particularly the junction, is likely to break. Toprevent the break, the metal pipe may be thickened, or may be made of amaterial having higher strength.

When the pipe is thickened, part of the pipe on which the stress is notconcentrated, i.e., part of the pipe which is less likely to break, isalso thickened. This unnecessarily increases weight of the metal pipe.Even when the material is changed, the weight of the metal pipe hardlychanges.

As disclosed by Patent Document 1, for example, reduction of the weightof the metal pipe has been attempted while preventing the break bymaking part of the metal pipe on which the stress tends to concentratethicker than the other part. According to Patent Document 1, a metalpipe of a uniform thickness is heated to a high temperature of about1200° C. to reduce deformation resistance, and then the metal pipe ispressed in an axial direction thereof using a die and a mandrel to forma thick portion in the metal pipe.

CITATION LIST

-   [Patent Document 1] Japanese Patent Publication No. H08-10889

SUMMARY OF THE INVENTION Technical Problem

According to Patent Document 1, the metal pipe needs to be heated tohigh temperature. The heating takes time, and reduces productionefficiency. Further, the processing consumes much energy.

When the metal pipe processed at high temperature is cooled to roomtemperature, thermal contraction does not always occur uniformly. Thisreduces dimensional accuracy of the metal pipe.

In view of the foregoing, the present invention has been achieved. Thepresent invention is concerned with providing a metal pipe having athick portion with high production efficiency, high precision, andreduced energy consumption.

Solution to the Problem

In view of the above concern, a first aspect of the invention isdirected to a method for processing the metal pipe by providing a stressconcentration portion in an axial center of the metal pipe, applying anaxial pressure to the metal pipe to concentrate stress on the stressconcentration portion to deform the stress concentration portion, andforming a thick portion from the deformed portion as a starting point.

The first aspect of the invention provides a method for processing ametal pipe to make part of the metal pipe in an axial direction thereofinto a thick portion thicker than a different portion, the methodincludes: forming a stress concentration portion in an axial center ofthe metal pipe to concentrate a stress on the stress concentrationportion when an axial pressure is applied to the metal pipe; setting themetal pipe in an outer die for holding an outer peripheral surface ofthe metal pipe; and forming the thick portion by inserting an inner diein the metal pipe to provide space for forming the thick portion betweenthe inner die and the outer die, and applying the axial pressure to themetal pipe to concentrate the stress on the stress concentration portionto deform the stress concentration portion after the forming of thestress concentration portion and the setting of the metal pipe, therebyforming the thick portion from the deformed stress concentration portionas a starting point.

With this configuration, the stress is concentrated on the stressconcentration portion of the metal pipe when the pressure is applied tothe metal pipe. Thus, unlike the conventional example, deformation ofthe metal pipe starts without reducing the deformation pressure byheating the pipe. The deformation of the metal pipe occurs in the spacebetween the outer die and the inner die from the first deformed portionas the starting point. Thus, the thick portion is formed in the metalpipe between the outer die and the inner die.

According to a second aspect of the invention related to the firstaspect of the invention, a shoulder is formed in the metal pipe in theforming of the stress concentration portion to use the shoulder as thestress concentration portion.

With this configuration, stress concentration can reliably be causedwhen the axial pressure is applied to the metal pipe by forming theshoulder.

According to a third aspect of the invention related to the secondaspect of the invention, the shoulder is formed by expanding the metalpipe in the forming of the stress concentration portion.

This configuration allows easy provision of the shoulder by theexpansion.

A fourth aspect of the invention is directed to a device for processinga metal pipe, wherein a stress concentration portion is formed in anaxial center of the metal pipe using a stress concentration portionformation device, and an inner die is pressed in the axial direction ofthe metal pipe by a drive to apply an axial pressure to an end of themetal pipe.

The fourth aspect of the invention provides the device for processingthe metal pipe to make part of the metal pipe in an axial directionthereof into a thick portion thicker than a different portion, thedevice including: a stress concentration portion formation device forforming a stress concentration portion in an axial center of the metalpipe to concentrate a stress on the stress concentration portion when anaxial pressure is applied to the metal pipe; an outer die for holding anouter peripheral surface of the metal pipe; an inner die which isinserted in the metal pipe to provide space for forming the thickportion between the outer die and the inner die, and comes into contactwith an end of the metal pipe; and a drive which presses the inner diein the axial direction of the metal pipe to apply an axial pressure tothe end of the metal pipe.

With this configuration, the metal pipe provided with the stressconcentration portion can be held by the outer die, the inner die can beinserted in the metal pipe, and the axial pressure can be applied to theend of the metal pipe by the drive. Thus, the stress is concentrated onthe stress concentration portion of the metal pipe to deform the stressconcentration portion, and the thick portion is formed between the outerdie and the inner die from the deformed stress concentration portion asa starting point.

According to a fifth aspect of the invention related to the fourthaspect of the invention, the inner die includes a first die which isinserted in part of the metal pipe except for part of the metal pipe forforming the thick portion, and a second die which is separated from thefirst die, and is inserted in the part of the metal pipe for forming thethick portion, and the second die is driven by the drive.

With this configuration, the pressure is applied to the metal pipe withthe first die being inserted in the part of the metal pipe except forthe part of the metal pipe for forming the thick portion, and the seconddie separated from the first die being inserted in the part of the metalpipe for forming the thick portion. Thus, the first die inserted in thepart of the metal pipe except for the part of the metal pipe for formingthe thick portion can be kept stationary, and the first die does notaxially rub an inner peripheral surface of the part of the metal pipeexcept for the part for forming the thick portion. This can prevent theinner peripheral surface of the metal pipe from damage or deformationdue to the rubbing of the inner die in the axial direction.

A sixth aspect of the invention is directed to a metal pipe which isprocessed by the method according to any one of the first to thirdaspects of the invention.

Advantages of the Invention

According to the first aspect of the invention, the stress concentrationportion is formed in the axial center of the metal pipe, and the metalpipe is set to the outer die. Then, the inner die is inserted in themetal pipe, and the axial pressure is applied to the metal pipe toconcentrate the stress on the stress concentration portion and deformthe stress concentration portion. The thick portion is formed from thedeformed stress concentration portion as the starting point. Thus, thereis no need to heat the metal pipe to high temperature to reduce thedeformation resistance. This can increase production efficiency anddimensional accuracy of the metal pipe, and can reduce energyconsumption. Thus, the metal pipe which is lightweight, and has arequired strength in a required portion can be obtained with highprecision at low cost.

According to the second aspect of the invention, the shoulder is formedin the metal pipe, and the shoulder is used as the stress concentrationportion. Thus, stress concentration can reliably be caused when thepressure is applied, and the thick portion can be formed as desired.

According to the third aspect of the invention, the shoulder is formedby expanding the metal pipe. Thus, the shoulder can easily be obtained,and the metal pipe can be obtained at lower cost.

According to the fourth aspect of the invention, like the first aspectof the invention, the metal pipe can be provided with the thick portionwithout heating the metal pipe to high temperature to reduce thedeformation resistance. This can increase production efficiency anddimensional accuracy, and can reduce energy consumption. Thus, the metalpipe which is lightweight, and has a required strength in a requiredportion can be obtained with high precision at low cost.

According to the fifth aspect of the invention, the first die isinserted in the part of the metal pipe except for the part of the metalpipe for forming the thick portion, the second die is inserted in thepart of the metal pipe for forming the thick portion, and the second dieis driven by the drive. This can prevent the inner peripheral surface ofthe metal pipe from damage or deformation in thickening the metal pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a metal pipe according toa present embodiment fixed to a flange component.

FIG. 2 is a cross-sectional view illustrating an enlargement of anupstream part of the metal pipe.

FIG. 3 is a cross-sectional view of a processing device.

FIG. 4 is a cross-sectional view of a metal pipe before processing.

FIG. 5 is a cross-sectional view of the metal pipe after expansion.

FIG. 6 is a cross-sectional view of an outer die.

FIG. 7 is a side view of a first die.

FIG. 8 is a side view of a second die.

FIG. 9 is a view corresponding to FIG. 3 illustrating the expanded metalpipe set in a die.

FIG. 10 is a view corresponding to FIG. 3 illustrating the metal pipeprovided with a thick portion.

FIG. 11 is a cross-sectional view of the metal pipe after processing.

FIG. 12 is a side view of an inner die according to an alternative.

FIG. 13 is a cross-sectional view illustrating a metal pipe providedwith a stress concentration portion according to an alternative.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described in detail belowwith reference to the drawings. The embodiment will be set forth merelyfor the purposes of preferred examples in nature, and is not intended tolimit the scope, applications, and use of the invention.

FIG. 1 shows a metal pipe 1 of an embodiment of the present inventionfixed to a flange component 2. The metal pipe 1 constitutes a pipe of anexhaust manifold, which is part of an exhaust system of an internalcombustion system mounted in an automobile (not shown).

In the description of the present embodiment, an upstream part of themetal pipe 1 in a direction of an exhaust stream will be simply referredto as an “upstream part”, and a downstream part of the metal pipe 1 inthe direction of the exhaust stream will be simply referred to as a“downstream part” for the sake of easy description.

As shown in FIG. 2, the upstream part of the metal pipe 1 is inserted ina through hole 2 a formed in the flange component 2. An outer peripheralsurface of the upstream part of the metal pipe 1 is entirely welded to arim of the through hole 2 a of the flange component 2. A referencecharacter C shown in FIGS. 1 and 2 designates weld bead. The flangecomponent 2 forms a flange on the metal pipe 1, and is able to beconnected to the other component.

The metal pipe 1 is made of a steel pipe. As shown in FIG. 4, the metalpipe 1 before processing has a uniform thickness from an end to theother end thereof. The thickness of the metal pipe of the presentembodiment is 1.2 mm.

As shown in FIG. 1, a predetermined portion of the upstream part of themetal pipe 1 is formed as a thick portion 10 which is thicker than theother portion. The thick portion 10 is obtained by a processing methoddescribed later. The thick portion 10 is 2.0 mm in thickness.

An outer diameter of part of the metal pipe 1 corresponding to the thickportion 10 is larger than an outer diameter of the other part. Thus, thethick portion 10 is thickened to bulge outward from the metal pipe 1. Asshown in FIG. 2, part of the outer peripheral surface of the metal pipe1 at a start end of the thick portion 10 (a downstream end of the thickportion 10) is tapered to increase the outer diameter toward theupstream side to form a diameter-increasing portion 10 a. Through thediameter-increasing portion 10 a, the thickness of the metal pipe 1gradually increases.

A surface 10 b, which is part of the outer peripheral surface of themetal pipe 1 upstream of the diameter-increasing portion 10 a, extendssubstantially parallel to an axis of the pipe. An axial dimension of thediameter-increasing portion 10 a is sufficiently smaller than an axialdimension of the surface 10 b.

A surface 1 a, which is part of the outer peripheral surface of themetal pipe 1 except for the thick portion 10, extends substantiallyparallel to the axis of the pipe. A curved surface 1 b is formed betweenthe surface 1 a and the diameter-increasing portion 10 a, and thesurface 1 a and the diameter-increasing portion 10 a are continuouslyconnected through the curved surface 1 b. A radius of curvature of thecurved surface 1 b is 5 mm to 15 mm, both inclusive. The provision ofthe curved surface 1 b reduces the occurrence of stress concentration.When the radius of curvature of the curved surface 1 b is smaller than 5mm, a stress generated near the curved surface 1 b increases, therebyreducing reliability of the metal pipe 1. When the radius of curvatureof the curved surface 1 b is larger than 15 mm, a total length of themetal pipe 1 increases, thereby reducing ease of layout.

An angle α formed by an extension line of the surface 1 a of the metalpipe 1 (a dotted line shown in FIG. 2) and the diameter-increasingportion 10 a is set to 5° to 25°, both inclusive. When the angle α issmaller than 5°, part of the metal pipe where the thickness graduallyincreases (a gradually thickening part) becomes longer, thereby reducingthe ease of layout of the metal pipe 1. When the angle α exceeds 25°, astress is concentrated too much on part A, and the stress generated atpart A exceeds a stress generated at part B. Thus, the angle ispreferably set in this range.

A large diameter portion lc having a larger diameter than a downstreamportion is formed in an inner peripheral surface of an upstream end ofthe metal pipe 1. Part of the inner peripheral surface downward of thelarge diameter portion 1 c is tapered to form a diameter-reducingportion 1 d which has an inner diameter reduced toward the downstreamside, and is continuous with the large diameter portion 1 c. With theprovision of the diameter-reducing portion 1 d, the thickness of theupstream end of the metal pipe 1 is gradually reduced toward the largediameter portion 1 c. Since the diameter-reducing portion 1 d is formedto gradually change the thickness, stress concentration is less likelyto occur. An axial dimension of the large diameter portion 1 c and anaxial dimension of the diameter-reducing portion 1 d are significantlysmaller than an axial dimension of the thick portion 10.

A method for processing the metal pipe 1 to provide the thick portion 10will be described below.

The metal pipe 1 has a round cross-section before processing. Theunprocessed metal pipe 1 has a thickness of 1.2 mm, and an outerdiameter of about 40 mm. The unprocessed metal pipe 1 has a length ofabout 120 mm. The thickness, the outer diameter, and the length of theunprocessed metal pipe 1 described above are merely examples, and arenot limited to the above examples.

As shown in FIG. 5, the metal pipe 1 is expanded. Specifically, part ofthe metal pipe 1 for forming the thick portion 10 is expanded. Thus, theexpansion starts from the part for forming the diameter-increasingportion 10 a (shown in FIG. 2), and is finished at the upstream end ofthe metal pipe 1.

The expansion is performed using a known expansion device 15. Thus, ashoulder 100 is formed in an axial center of the metal pipe 1. Theshoulder 100 is a stress concentration portion of the present invention.In this way, the stress concentration portion is formed, and theexpansion device 15 is a stress concentration portion formation device.

The expanded metal pipe 1 is set in an outer die 20 as shown in FIG. 9.The outer die 20 can be divided into a first member 21 and a secondmember 22. The first member 21 is provided with a recessed surface 21 awhich extends along half of the outer peripheral surface of the metalpipe 1 in a circumferential direction. The second member 22 is providedwith a recessed surface 22 a which extends along the other half of theouter peripheral surface of the metal pipe 1 in the circumferentialdirection. The first and second members 21 and 22 are fastened andintegrated by a fastening member which is not shown with the recessedsurfaces 21 a and 22 a facing to each other. In this state, the recessedsurfaces 21 a and 22 a form a through hole 20 a extending in thevertical direction. An inner diameter of an upper half of the throughhole 20 a is the same as an outer diameter of the expanded portion ofthe metal pipe 1, and an inner diameter of a lower half of the throughhole 20 a is the same as an outer diameter of part of the metal pipe 1except for the expanded portion.

As shown in FIG. 3, a lower end of the outer die 20 is fixed to a baseplate 23. A through hole 23 a communicating with a lower end of thethrough hole 20 a of the outer die 20 is formed in the base plate 23 topenetrate the base plate 23 in the vertical direction. An inner diameterof the through hole 23 a of the base plate 23 is smaller than an innerdiameter of the lower half of the through hole 20 of the outer die 20.As shown in FIG. 9, a downstream end of the metal pipe 1 is brought downfrom above to be contact with a rim of the through hole 23 a of the baseplate 23. The metal pipe 1 is supported by the base plate 23 in thisstate. The base plate 23 is provided with a recess 23 b in which a lowerend of the outer die 20 is fitted.

When the metal pipe 1 is inserted in the outer die 20 with the expandedportion facing upward, the entire outer peripheral surface of the metalpipe 1 is held by the outer die 20. Thus, the pipe is set.

An inner die 25 is inserted in the metal pipe 1 after the expansion andthe setting of the pipe.

Either of the expansion and the setting of the pipe may be performedfirst. Specifically, the expansion may be performed after the setting ofthe pipe.

As shown in FIGS. 3 and 9, the inner die 25 includes a first die 27which is inserted in part of the metal pipe 1 except for part of themetal pipe 1 for forming the thick portion 10, a second die 28 which isseparated from the first die 27, and is inserted in the part of themetal pipe 1 for forming the thick portion 10, and a spring 29 providedbetween the first and second dies 27 and 28.

The first die 27 is in the shape of a vertically extending column. Asshown in FIG. 7, an outer diameter of the first die 27 is substantiallythe same from an upper end to a lower end thereof, and is substantiallythe same as an inner diameter of an unexpanded part of the metal pipe 1.Thus, as shown in FIG. 3, the lower end of the first die 27 is insertedin the through hole 23 a of the base plate 23. The first die 27 islonger than the unexpanded part of the metal pipe 1.

As shown in FIG. 7, a tapered portion 27 a is formed at the lower end ofthe first die 27. The first die 27 is provided with a center hole 27 bextending along an axis thereof. The center hole 27 b is formed topenetrate the first die 27 in the vertical direction. A receiver portion27 c protruding inward of the center hole 27 b is formed in part of aninner peripheral surface of the center hole 27 b near a vertical centerthereof to receive a lower end of the spring 29.

The second die 28 is also in the shape of a column. As shown in FIG. 8,a first small diameter portion 28 a is formed in a lower end of thesecond die 28. An outer diameter of the first small diameter portion 28a is substantially the same as the outer diameter of the first die 27. Atapered portion 28 b which has a diameter reducing downward, and iscontinuous from the first small diameter portion 28 a is formed upwardof the first small diameter portion 28 a of the second die 28. A secondsmall diameter portion 28 c is formed upward of the tapered portion 28 bof the second die 28. The second small diameter portion 28 c has alarger diameter than the first small diameter portion 28 a.

An outer diameter of a body 28 d of the second die 28 upward of thesecond small diameter portion 28 c is substantially the same as an innerdiameter of an upper part of the through hole 20 a of the outer die 20.A step 28 e is formed between the body 28 d and the second smalldiameter portion 28 c. The upstream end of the metal pipe 1 is fitted inthe step 28 e.

The second die 28 is provided with a threaded hole 28 f extending alongan axis thereof. The threaded hole 28 f is opened in an upper endsurface of the second die 28. A spring insertion hole 28 g is formed ina lower part of the second die 28 in which the spring 29 is inserted. Anupper end of the spring 29 abuts a bottom of the spring insertion hole28 g.

As shown in FIG. 3, a plate 30 is fixed to an upper end of the seconddie 28 with a bolt 31. A bolt insertion hole 30 a in which the bolt 31is inserted is formed in a center of the plate 30. The bolt 31 insertedin the bolt insertion hole 30 a is screw-fitted to the threaded hole 28f of the second die 28.

A drive 33 is coupled to the plate 30. The drive 33 is provided to movethe second die 28 in the vertical direction.

In inserting the inner die 25 in the metal pipe 1, the first die 27 isinserted in the metal pipe 1, and then the second die 28 is inserted. Asthe second die 28 is inserted, the spring 29 is pressed downward,thereby pressing the first die 27 downward. Thus, the lower end of thefirst die 27 is inserted in the through hole 23 a of the base plate 23.

When the inner die 25 is inserted in the metal pipe 1, space S forforming the thick portion 10 is provided between the inner die 25 andthe outer die 20 as shown in FIG. 9.

When the second die 28 is pushed downward by the drive 33 as indicatedby an outline arrow in FIG. 9, a downward pressure is acted on the upperend of the metal pipe 1. The pressed metal pipe 1 generates acompressive stress. However, since the cross section of the metal pipe 1is not uniform, but is partially changed by forming the shoulder 100through the expansion, high stress is concentrated on the shoulder 100.As a result, the shoulder 100 starts to be deformed. The provision ofthe shoulder 100 makes a force required to start the deformation smalleras compared with the case where the shoulder 100 is not formed. Thus,unlike the conventional examples, the metal pipe 1 can be deformedwithout reducing the deformation pressure by heating the pipe.Specifically, there is no need to heat the pipe to a temperature ofabout 300° C. or higher for easy processing, i.e., so-called coldprocessing can be performed.

When the second die 28 is further moved downward by the drive 33,deformation of part of the metal pipe upward of the shoulder 100 startsfrom the deformed shoulder 100. The deformation occurs in the space Sbetween the outer die 20 and the inner die 5, and the deformed part ofthe metal pipe 1 is molded between the inner peripheral surface of theouter die 20 and the outer peripheral surface of the inner die 25 tobecome the thick portion 10. FIG. 11 shows the processed metal pipe 1.

A thickness of part of the metal pipe 1 except for the expanded part isnot greatly changed for the following reasons. Specifically, a smallclearance is provided between the outer die 20 and the outer peripheralsurface of the metal pipe 1, and between the inner die 25 and the innerperipheral surface of the metal pipe 1 in view of moldability. Informing the shoulder 100 as the stress concentration portion, thepressure is used to form the shoulder 100. Thus, a stress caused in partof the metal pipe 1 downward of the shoulder 100 is reduced, and changein thickness of the part of the metal pipe 1 except for the expandedpart is reduced.

The drive 33 moves the second die 28 only, and the first die 27 is keptstationary. Thus, the first die 27 does not axially rub part of theinner peripheral surface except for part thereof for forming the thickportion 10. This can prevent the part of the metal pipe 1 except for thepart for forming the thick portion 10 from damage or deformation.

The expansion device 15, the outer die 20, the inner die 25, the baseplate 23, the plate 30, and the drive 33 constitute a processing deviceof the present invention.

As shown in FIG. 1, the upstream end of the metal pipe 1 processed asdescribed above is inserted in the through hole 2 a of the flangecomponent 2, and is welded to the rim of the through hole 2 a of theflange component 2.

When a downward load is applied to the downstream end of the metal pipe1 with the metal pipe 1 fixed to the flange component 2, a stress isgenerated in every part of the metal pipe. At this time, parts enclosedwith circles A and B in FIG. 2 tend to experience a high stress. Sincethe angle α formed between the diameter-increasing portion 10 a and thesurface 1 a of the metal pipe 1 is set to 25° or smaller, the stresscaused at the part enclosed with circle A is smaller than the stresscaused at part B. Specifically, as compared with the stress caused atthe welded part of the metal pipe 1 (the part enclosed with circle B), astress caused at the other part (the part enclosed with circle A) issmaller. Thus, in use, the other part does not break earlier than thewelded part.

According to the present embodiment described above, the metal pipe 1including the shoulder 100 formed in the axial center of the metal pipe1 is set in the outer die 20, and an axial pressure is then applied tothe metal pipe 1 with the inner die 25 inserted in the metal pipe 1 toconcentrate the stress on the shoulder 100 to deform the shoulder 100.Then, the thick portion 10 is formed from the deformed portion as astarting point. Thus, unlike the conventional examples, there is no needto heat the metal pipe 1 to high temperature to reduce the deformationresistance. This can improve production efficiency and dimensionalaccuracy, and can reduce energy consumption. Thus, the metal pipe whichis lightweight, and has a required strength in a required portion can beobtained with high precision at low cost.

The shoulder 100 is formed in the metal pipe 1, and the shoulder 100 isused as the stress concentration portion. Thus, the stress concentrationcan reliably be caused when the pressure is applied, and the thickportion 10 can be formed as desired.

Since the shoulder 100 is formed by expanding the metal pipe 1, theshoulder 100 can easily be obtained, and the metal pipe 1 can beobtained at lower cost.

In the above embodiment, the inner die 25 is configured to be dividableinto the first die 27 and the second die 28. However, the inner die 25is not limited thereto, and the first die 27 and the second die 28 maybe integrated as those of an alternative shown in FIG. 12. The inner die25 of the alternative is also provided with a step 25 a in which theupstream end of the metal pipe 1 is fitted. The integrated inner die 25of this alternative can reduce the parts count, and can reduce the cost.Further, the metal pipe 1 can be molded with higher precision becausemisalignment between the first die 27 and the second die 28 does notoccur.

In the above-described embodiment, the shoulder 100 is formed in themetal pipe 1, and the shoulder 100 is used as the stress concentrationportion. However, the stress concentration portion is not limited to theshoulder. For example, as shown in FIG. 13, a protrusion 101 may beformed as the stress concentration portion. The protrusion 101 may bereplaced with a recess. Alternatively, a thin portion may be formed asthe stress concentration portion.

In the above-described embodiment, the thick portion 10 is formed tobulge outward from the outer peripheral surface of the metal pipe 1.However, the thick portion 10 is not limited thereto, and may be formedto bulge inward from the inner peripheral surface of the metal pipe 1.

Materials of the metal pipe 1 are not limited as long as they can beused for deformation processing, and various types of materials can beused.

The metal pipe 1 can be used not only as a part of exhaust systems ofautomobiles, but can also be used as, e.g., air pipes, liquid pipes,etc.

The diameter of the metal pipe 1 is not particularly limited. Forexample, the diameter may be about several tens cm.

INDUSTRIAL APPLICABILITY

As described above, the present invention can be applied to, forexample, metal pipes constituting an exhaust system of an automobile.

DESCRIPTION OF REFERENCE CHARACTERS

-   1 Metal pipe-   2 Flange component-   10 Thick portion-   15 Expansion device (stress concentration portion formation device)-   20 Outer die-   25 Inner die-   27 First die-   28 Second die-   29 Spring-   33 Drive-   100 Shoulder (stress concentration portion)-   101 Protrusion (stress concentration portion)

1. A method for processing a metal pipe to make part of the metal pipein an axial direction thereof into a thick portion thicker than adifferent portion, the method comprising: forming a stress concentrationportion in an axial center of the metal pipe to concentrate a stress onthe stress concentration portion when an axial pressure is applied tothe metal pipe; setting the metal pipe in an outer die for holding anouter peripheral surface of the metal pipe; and forming the thickportion by inserting an inner die in the metal pipe to provide space forforming the thick portion between the inner die and the outer die, andapplying the axial pressure to the metal pipe to concentrate the stresson the stress concentration portion to deform the stress concentrationportion after the forming of the stress concentration portion and thesetting of the metal pipe, thereby forming the thick portion from thedeformed stress concentration portion as a starting point.
 2. The methodof claim 1, wherein a shoulder is formed in the metal pipe in theforming of the stress concentration portion to use the shoulder as thestress concentration portion.
 3. The method of claim 2, wherein theshoulder is formed by expanding the metal pipe in the forming of thestress concentration portion.
 4. A device for processing a metal pipe tomake part of the metal pipe in an axial direction thereof into a thickportion thicker than a different portion, the device comprising: astress concentration portion formation device for forming a stressconcentration portion in an axial center of the metal pipe toconcentrate a stress on the stress concentration portion when an axialpressure is applied to the metal pipe; an outer die for holding an outerperipheral surface of the metal pipe; an inner die which is inserted inthe metal pipe to provide space for forming the thick portion betweenthe outer die and the inner die, and comes into contact with an end ofthe metal pipe; and a drive which presses the inner die in the axialdirection of the metal pipe to apply an axial pressure to the end of themetal pipe.
 5. The device of claim 4, wherein the inner die includes afirst die which is inserted in part of the metal pipe except for part ofthe metal pipe for forming the thick portion, and a second die which isseparated from the first die, and is inserted in the part of the metalpipe for forming the thick portion, and the second die is driven by thedrive.
 6. A metal pipe processed by the method of claim
 1. 7. A metalpipe processed by the method of claim
 2. 8. A metal pipe processed bythe method of claim 3.